It clearly depends on School. Yet you can go contact the  schools near you. And coming back to second question its also up to the will of school to enroll you for the science stream.

Dear student,

As you have mentioned that you have scored 78% in your ICSE board and you are wanting to take admission in cbse board in class. 11 so let me tell you that it totally depends on that specific school on what basis they want to take admission.

They can also take entrance for taking admission and also on the basis of your 10th scores. So I would recommend you to contact the school authorities regarding this.